We have isolated the polypeptide hormone relaxin from the ovaries of pregnant pigs and pregnant rats. Our structural studies indicate that porcine and rat relaxin have molecular weights of approximately 6,000 and consist of two nonidentical chains (A and B) which are linked by disulfide bonds. The amino acid compositions and immunological properties of pig and rat relaxin are similar but not identical. The physiology of relaxin in the pig and rat remains poorly understoood. We have developed sensitive radioimmunoassays for porcine and rat relaxin and have employed these radioimmunoassays for the determination of plasma relaxin levels throughout pregnancy in both of these species. We plan to determine plasma relaxin levels in pregnant pigs and rats under experimental conditions designed to provide insight into (1) factors which influence relaxin secretion during pregnancy and (2) factors which influence luteolysis during late pregnancy. We plan to determine ovarian relaxin levels in pregnant and pseudopregnant rats under experimental conditions designed to identify the factor(s) which influence relaxin biosynthesis during pregnancy. We then plan to isolate and characterize the factor(s) which stimulates relaxin biosynthesis. We plan to conduct experiments aimed at gaining a better understanding of the physiological effects of relaxin on the uterus of rats and pigs during late pregnancy. We propose to determine (1) the effects of relaxin on myometrial contractions and electrical activity in rats by employing conditions designed to provide a better understanding of the way in which relaxin interacts with other hormones or pharmacological agents which influence uterine function during late pregnancy and parturition and (2) the physiological consequences of deprivation of endogenous relaxin during pregnancy in the rat and pig.